This invention relates in general to the blades employed in turbo-machinery and, more particularly, to blade structures with improved cooling.
Turbine blades employed in gas turbine engines include a leading edge and a trailing edge. The leading edge is the blade surface which is first contacted by the working medium gases in the turbo-machine. The trailing edge is the blade surface which is last contacted by the working medium gases as they pass by the blade.
The temperatures within turbine engines may exceed 2500 degrees F. and thus cooling of turbine blades becomes very important in terms of engine longevity. Without cooling, turbine blades would rapidly deteriorate. Clearly, improved cooling techniques for turbine blades are very desirable. Conventional turbine blades have internal cavities into which cooling air is pumped to cool the blade. Much effort has been devoted by those skilled in the blade cooling arts to devise improved geometries for the internal cavities within turbine blades in order to enhance cooling.
For example, it is known to use a "cold bridge cooling circuit" to cool a turbine blade. In such a cold bridge cooling circuit, cooling air is supplied directly through an inlet cavity and the cooling air impinges on the leading edge of the blade after minimum heat pickup in the blade airfoil. Unfortunately this cooling technique causes an increase in the temperature difference across the leading edge of the blade. This results in increased thermal stresses in the blade's leading edge which reduces blade life, especially during transient operation where the temperature difference can be amplified.
It is also known to use "warm bridge cooling circuits" to provide cooling to turbine blades. In a warm bridge cooling circuit, the cool inlet air passes through passages in the interior of the blade and warms up as it travels through the passages before impinging on the leading edge of the blade. Advantageously, the temperature difference across the leading edge is much less with this approach. Consequently, lower thermal stresses result in the blade leading edge and the life of the blade is enhanced.
The warm bridge cooling circuit makes efficient use of cooling flow since the flow is able to internally cool the blade over much of the blade mid-span before flowing out radial leading edge cooling holes to film cool the blade airfoil externally. Unfortunately, a major disadvantage of using a warm bridge cooling circuit for this application is "backflow margin". As air flow travels through the internal passages of the blade, pressure losses due to turns and turbulence promoters cause the cooling flow pressure to drop to a level such that gas ingestion into the blade leading edge is no longer reliably preventable. This undesired condition is referred to as backflow. One approach for providing more backflow margin is to increase the inlet pressure of the cooling air which is supplied to the blade. This approach is not always feasible because the increase in supply pressure can increase cooling flow leakages to an undesired level.